Conventionally, an electrophotographic image forming apparatus has been widely used as an image forming apparatus such as a copying machine, a printer, or a facsimile device.
In the electrophotographic image forming apparatus, a charging device electrically charges a photosensitive surface of a photosensitive drum. The electrically charged photosensitive surface is then subjected to an exposure process using an exposure device, so that an electrostatic latent image is formed on the photosensitive surface of the photosensitive drum. Then, a developing device develops the electrostatic latent image having been formed, so as to form a toner image (visible image) on the photosensitive surface. After this toner image is transferred to a sheet (recording material such as paper, OHP (Overhead Projector) recording paper, etc.) by a transfer device, the toner image is fixed by using a fixing device.
The fixing device fixes toner on a recording medium such as recording paper by applying heat and pressure on the recording medium. In general, a fixing device having a fixing (heating) roller and a pressure roller is widely used. Here, this type of fixing device is referred to as a pair-roller fixing device. In the pair-roller fixing device, a sheet having thereon an unfixed toner image is transferred through a nip region between the fixing and pressure rollers, thereby fixing the unfixed toner image on the sheet.
Incidentally, if the heat from the fixing device is not exhausted out of the image forming apparatus, the heat built up in the image forming apparatus would give a negative effect to other devices in the image forming apparatus. In view of that problem, an exhaust fan is often provided, for the purpose of discharging the heat in the image forming apparatus. For example, Japanese Unexamined Patent Publication No. 150026/2003 (Tokukai 2003-150026; published on May 21, 2003) discloses an image forming apparatus including an expansible silencer is provide in an air-blowing path for a heat-discharging fan, for the purpose of reducing noise produced by the heat-discharging fan.
Further, Japanese Unexamined Patent Publication No. 258904/1994 (Tokukaihei 6-258904; published on Sep. 16, 1994) discloses a technology which (i) reduces a necessary cooling ability that a fan should have; or (ii) eliminating the need of a fan itself. In order to achieve this, the technology uses a temperature adjusting device which absorbs heat from a high temperature part in the main body of the image forming apparatus, and discharges the absorbed heat to a low-temperature part.
Further, Japanese Unexamined Patent Publication No. 63019/1996 (Tokukaihei 8-63019; published on Mar. 26, 1996) discloses a configuration in which a pressure roller is surrounded by a thermal conductor whose surface (inner surface) facing toward the pressure has a higher heat absorption coefficient than a surface (outer surface) opposite thereto. In this configuration, the inner surface of the heat conductive member has a higher heat absorption coefficient, so that the inner surface of the heat conductive member absorbs the heat from the pressure roller, and evenly distributes the heat through out the thermal conductor. The heat evenly distributed through out the thermal conductor is then radiated to the pressure roller. Further, the outer surface of the heat conductive member has a smaller heat absorption coefficient. This restrains the heat generated by the pressure roller from being dissipated out of the image forming device. In this way, the configuration disclosed in Tokukaihei 8-63019 (i) efficiently utilizes the heat; (ii) restrains an increase in an internal temperature in the device; and (iii) reducing a necessary ability that an heat-discharging fan should have.
However, the above conventional technology causes the following problems when applied to an image forming apparatus in which powder developer (toner) is used for visualizing the electrostatic latent image being formed on a photoreceptor by an electrophotographic recording method or an electrostatic recording method, which is provided with a venting-out fan for venting, to the outside, the heat in the image forming apparatus. Namely, toner floating in an image forming apparatus will be dissipated into the surrounding environment, whereby the environment surrounding the image forming apparatus gets dirty with the toner.
Particularly, in recent years, a particle diameter of toner has been reduced for the purpose of improving a quality of an image, and a printing speed has been accelerated. This makes it easier to cause dispersing or floating the toner in the image forming apparatus. Thus, the floating toner contained in the discharged air from the image forming apparatus is particularly becoming an issue.
Further, the foregoing Tokukaihei 6-258904 describes that the use of the fan itself is eliminated by providing the temperature adjusting device for discharging the heat in the high temperature portion of the image forming apparatus to the low temperature portion. However, if the heat in the high temperature part is not sufficiently discharged, there is a possibility that the internal temperature of the apparatus increases, and negatively affects the members in the image forming apparatus. Further, even if the apparatus is not provided with a fan, the toner floating in the apparatus may be let out of the apparatus through a recording material transporting path or the like. Further, even if it is possible to eliminate the need for a fan, the floating toner in the image apparatus may adhere to an optical element or the like of an exposure device. This deteriorates an image quality.